It is frequently useful in photoelectric sensing to obtain an analog voltage or current signal that is a function of signal strength which, in turn, is a function of distance, reflectivity, and other factors. One application for such a signal is in the area of process control, where it is desirable to alter the speed of a motor which controls an unwind roll in response to the nearness of a loop of web material to the photoelectric sensor. Another application is in edge-guiding a web, where the analog signal represents the position of the web edge, and the signal is then used to control the edge-guide motors to keep the web centered. A third application is in controlling a robot or automatic guided vehicle, so that it slows down and eventually stops as it approaches a solid surface, such as a wall. Another typical application, which may become increasingly popular with the trend toward the use of programmable controllers, is to monitor the strength of the analog signal and generate a warning indication when the analog signal becomes marginal, but before the photoelectric sensor is rendered inoperative by dirt, dust, moisture, misalignment, or other factors.
The design and use of analog photoelectric sensors is well known, as characterized by the analog series of Multi-Beam sensors manufactured by Banner Engineering Corp., the Assignee of the present invention, and by similar products of other manufacturers. These analog photoelectric sensors are typically powered by low voltage DC (e.g., 12-28 v DC) or by AC line voltage (e.g., 120 v AC). Such sensors have an output which is generally a DC voltage (e.g., 0-10 v DC) or a DC current (e.g., 4-20 milliamps). Such an output requires a third wire (a DC signal line) in the case of DC sensors, or a third and fourth wire (a DC signal line and a DC ground) in the case of AC sensors.
The 4-20 milliamp current loop on which these sensor outputs operate has been widely accepted as the standard in the instrumentation and control industry. The use of a current for the variable parameter minimizes the effects of any transmission line resistance. Furthermore, using 4 milliamps as the lower limit means that the monitoring circuit may detect when the line is broken by sensing zero current.
The independent power supply for the sensors must be able to supply whatever current is required to run the sensor circuitry plus the analog output current. A typical requirement is 20 milliamps for the sensor and another 20 milliamps for the maximum output current. The separate power supply used by these sensors provides the current necessary to produce a light or, in other applications, an acoustic signal with sufficient range.
The third wire in these three-wire systems sources or sinks a variable current by "burning" or absorbing the appropriate amount of current to introduce on the current loop a current proportional to the measured parameter. The measured parameter may be, for example, the distance between a target and the sensor. The instrumentation circuitry coupled to the sensor receives the variable current and can thus produce control signals based upon the measured characteristic.
The independent power supply required by these three-wire sensors represents a significant cost when compared with the cost of a typical sensor. The power supply may actually cost as much as the sensor itself.
Therefore, there is a need in the industry for a sensor that operates directly from the power of a standard current loop so that an independent power supply is not required. In particular, there is a need for a two-wire photoelectric sensor that operates directly from an industry standard 4-20 milliamp current loop.